1. Field of the Invention
The present invention pertains to contacts, vias and other pathways formed in film layers of semiconductor devices, and to the lining or filling of apertures, such as vias, holes or trenches extending through one or more film layers on a semiconductor or other type of substrate, to create the pathway through the film layer. The invention enables the filling of high aspect ratio apertures (i.e., those having a ratio of height to width of from about 1:0 to about 12:0 and a width of as small as 1000 angstroms) and planarization of the deposited film layer at reduced temperatures as compared to prior art methods, and, in some cases, where traditional prior art methods cannot provide void free filling of the aperture and planarization of the resulting film layer in reasonable periods of time. In one sub-embodiment, the invention pertains to a carrier layer, to the structure and composition of the carrier layer, to methods for forming the carrier layer, and to the equipment used to practice the methods for forming the carrier layer, all of which enable the filling of high aspect ratio electrical contacts and planarization of the deposited film layer during the fabrication of integrated circuit devices.
2. Description of the Background Art
During the fabrication of integrated circuit devices on a substrate or wafer, it becomes necessary to fill holes, trenches or vias (i.e., apertures) in film layers, typically dielectric film layers, to create pathways, typically conductive pathways, through the film layer. Traditionally, the aperture filling has been provided by depositing a filling material layer over a previously deposited film layer, typically a dielectric film layer, having apertures therethrough. For example, a contact is created through a dielectric layer by depositing a conductor, such as aluminum, into an aperture in the dielectric layer. Where the material used to fill the aperture might be reactive with the materials over which it is deposited, such as where the filling material may interdiffuse with the material at the base of the aperture and result in a material having undesirable qualities, a barrier layer must be deposited in the aperture to line the aperture before the filling material is deposited. After the barrier layer is deposited, a conductive material is deposited thereover to form the contact. To enable further processing of the wafer, the conductive material used to fill the aperture must be planarized to reduce any significant variation in the height of the exposed portion of the film layer, which variation is a consequence of depositing a relatively uniform thickness filling material layer over a surface filled with the apertures.
Physical vapor deposition, or "PVD",is one known method of filling apertures useful in semiconductor device fabrication. In PVD, a target of the deposition material is exposed to a plasma and sputtered by ions from the plasma. The material sputtered from the target is deposited on a substrate. The deposited material forms a film layer on the substrate and is also used to fill the apertures. To provide a planarized deposited film layer, and to ensure the filling of the apertures, the film layer may be deposited at elevated temperatures to cause the conductive material to "reflow",thereby filling the apertures and creating a "planarized" layer, i.e., one having a relatively flat upper surface. Typically the reflow step is performed at substrate temperatures on the order of 480.degree. C. or above.
The filling of apertures using traditional sputtering methods has become difficult as the aperture aspect ratio has increased. Where the hole width equals the hole depth, i.e., at a 1:1 aspect ratio, traditional sputter deposition techniques may ensure conformal deposition of the target material on the sides and base of the hole and enable complete filling of the aperture. However, there is still a need, even with apertures having an aspect ratio of 1:1, to provide aperture filling at lower reflow temperatures. At higher aspect ratios, nominally at aspect ratios above about 2:1, the utility of traditional sputtering techniques for aperture filling is questionable. The reason is substantially a result of aperture and sputter geometries. Because the target particles sputtered from the target travel in linear paths, the base and lower portion of the walls of the aperture are blocked from those sputtered particles which are traveling transversely to the surface of the substrate. The deposit formed by the sputtered particles in high aspect ratio apertures tends to be very thick at the aperture opening and very thin at the base of the aperture. As the deposition of the sputtered particles continues, the material which is deposited at the opening of the aperture continues to build up during continued deposition. This deposit will increasingly block the target material from the base of the aperture wall. Eventually, the layer formed at the opening of the aperture can completely cover the aperture, preventing further deposition of the deposition material into the aperture. Therefore, to fill high aspect ratio apertures by sputter deposition, alternative methods must be used.
One alternative approach to sputtering which may be used to fill high aspect ratio apertures is coherent deposition, wherein a collimated supply of target material is deposited over the surfaces of the substrate at a low temperature, typically below 150.degree. C. to form a "seed" layer of material on the substrate. After the seed layer is formed, the substrate is moved to a second, noncoherent, sputtering chamber and the aperture is filled with a sputtered material which is deposited on the substrate while the substrate is maintained at a temperature of about 480.degree. C. or above. This high temperature causes the material being deposited on the substrate to reflow and thereby: 1) Fill the aperture; and, 2) result in a planarized film layer. The time required to fill the apertures and planarize the deposited film layer by the reflow technique is a function of the substrate temperature. The higher the substrate temperature, the faster the apertures are filled and the film layer planarized. However, if the substrate is too hot, the seed layer will coalesce into individual droplets and prevent the formation of a conformal film layer, or, previously deposited materials will change dimension or be thermally degraded. Where reflow techniques are used to fill apertures having a depth of 1.2 .mu. and an aspect ratio of approximately 1:1, and aluminum is being deposited as the aperture filling material, a substrate temperature of about 480.degree. C. can result in filled apertures and a planarized deposition layer in about three or four minutes. For 0.5 micron wide or smaller apertures, i.e., standard 1.2 .mu. deep apertures having aspect ratios of approximately 2:1 or higher, the reflow method has limited effectiveness. In particular, even using coherent deposition, higher aspect ratio holes may take too long to fill at an acceptable reflow temperature. Additionally, the collimator screens a substantial quantity of the deposition material and thereby reduces yield and throughput.
One additional known method of filling high aspect ratio apertures is to ionize at least a portion of the deposition material sputtered from the target and attract the ionized target material to the substrate. By ionizing the deposition material and electrically attracting it to the substrate, the deposition material reaching the substrate will be travelling perpendicular to the substrate. Thus, the deposition material will reach the base of the apertures and will not tend to collect on the upper reaches of the aperture wall.
For example, U.S. Pat. No. 5,178,739 to Barnes et al., issued Jan. 12, 1993 describes a sputter deposition system which includes a hollow, cylindrical sputter target disposed between an end sputter target and a substrate, all of which are contained in a vacuum chamber. A plurality of magnets are disposed outside the chamber to create intense plasma regions near the interior surface of the cylindrical target and thereby cause ionization of at least a portion of the material sputtered from the target. RF power is inductively coupled into the chamber through an interior rf coil located between the target and the substrate to sustain the plasma. The substrate is electrically biased to control the directionality and energy of the ionized sputtered material. The ions produced using the sputter deposition system are said to have low angular divergence after crossing the plasma sheath and therefore are said to be capable of uniformly filling high aspect ratio apertures such as holes. (A high aspect ratio hole is typically described as being a hole (via) having a height to width ratio of 1:1 or greater).
Although ionized deposition is a known way of filling high aspect ratio holes, the technique is significantly more expensive than traditional sputtering techniques because the deposition rate is slower, the power requirements are greater and the equipment is more expensive than traditional sputtering equipment. Notwithstanding these limitations, it is believed by those skilled in the art that the filling of very high aspect ratio holes, i.e., those above approximately 2:1, may only be accomplished using ionized deposition material, and that other techniques, such as coherent deposition followed by traditional sputter deposition and reflow cannot meet the needs of device fabricators as line widths continue to shrink.
An objective of the present invention is to provide a means which enables a conductive material, typically a metal or metal alloy, to fill a narrow, high aspect ratio, aperture without the formation of voids in the resulting deposit to provide a suitable integrated circuit electrical contact. The deposition of the metal or metal alloy is preferably carried out at substrate temperatures below 480.degree. C., so that the thermal budget for the process is low. Even more preferably, the sputtering of the metal or metal alloy is carried out at substrate temperatures below as little as about 350.degree. C. to reduce the initial and maintenance costs of the heating element used provide the substrate temperatures as well as the thermal budget for the process. These lower temperatures may also enable use of materials in IC fabrication which have lower thermal breakdown or creep temperatures but are useful to circuit designers, because deposition materials deposited on the substrate before the metal or metal alloy deposition step will experience lower processing temperatures than those of the prior art when the methods of the present invention are employed to provide filled apertures and a planarized film layer.